LOS ANGELES — A jury found Meta and YouTube negligent in the design or operation of their social media platforms, producing a bellwether verdict in the first lawsuit to take tech giants to trial for social media addiction.

The Los Angeles County Superior Court jury said that Meta’s and YouTube’s negligence were a substantial factor in causing harm to the plaintiff, identified in court by her initials, K.G.M., and that the companies failed to adequately warn users of the dangers of Instagram (Meta’s platform) and YouTube (which is owned by Google).

It awarded K.G.M. $3 million in compensatory damages, finding Meta 70% responsible for harm caused to the now 20-year-old plaintiff, and YouTube responsible for 30%.

The trial, which began last month in a Los Angeles County courtroom and included testimony from Mark Zuckerberg and other tech executives, was the first in a consolidated group of cases brought against Meta and other companies by more than 1,600 plaintiffs, including over 350 families and over 250 school districts.

Outside the courtroom, families who say their children were harmed by social media embraced as they celebrated the verdict, telling reporters they feel “vindicated.”

Spokespeople for Meta and Google said the companies disagree with the verdict and plan to appeal.

“Teen mental health is profoundly complex and cannot be linked to a single app,” a Meta spokesperson said. “We will continue to defend ourselves vigorously as every case is different, and we remain confident in our record of protecting teens online.”

José Castañeda, a spokesperson for Google, also said the case “misunderstands YouTube, which is a responsibly built streaming platform, not a social media site.”

In a joint statement, co-lead counsel for K.G.M. said the verdict is “a historic moment” for thousands of children and their families.

“But this verdict is bigger than one case,” the lawyers said. “For years, social media companies have profited from targeting children while concealing their addictive and dangerous design features. Today’s verdict is a referendum — from a jury, to an entire industry — that accountability has arrived.”

The jury decided on $2.1 million in punitive damages for Meta and $900,000 for YouTube, totaling $3 million. It’s a small fraction of the $1 billion in punitive damages the plaintiff’s counsel sought.

K.G.M.’s lead attorney, Mark Lanier, has said he hopes the proceedings produce transparency and accountability “so that the public can see that these companies have been orchestrating an addiction crisis in our country and, actually, the world.”

The plaintiff was a minor at the time of the incidents outlined in her lawsuit. K.G.M. testified in court that her nearly nonstop use of social media caused or contributed to depression, anxiety and body dysmorphia. It “really affected my self-worth,” she said last month.

Speaking about her social media use, K.G.M. testified that she felt she wanted to constantly be on the platforms and feared missing out if she wasn’t.

Attorneys for Meta and YouTube have disputed claims brought by the plaintiff, arguing their platforms aren’t purposefully harmful and addictive.

A spokesperson for Meta said K.G.M.’s “profound challenges” weren’t caused by social media and pointed to “significant emotional and physical abuse” that she experienced when she was younger.

In his closing argument, an attorney for YouTube said there wasn’t a single mention of addiction to that platform in K.G.M.’s medical records.

The verdict comes after jurors in a separate trial in New Mexico held Meta liable for failing to protect children from online predators and sexual exploitation on Facebook and Instagram.

The New Mexico jury found Tuesday that Meta violated the state’s consumer protection laws and ordered it to pay $375 million in civil penalties. Meta has said it disagrees with the verdict and plans to appeal.

In Los Angeles, deliberations took longer, wrapping up after nearly 44 hours over nine days. The jurors had told Judge Carolyn B. Kuhl that they were having trouble coming to a consensus on one defendant.

Social media companies have historically been shielded by Section 230, a provision added to the Communications Act of 1934 that says internet companies aren’t liable for the content users post.

K.G.M.’s lawsuit was the first civil action seeking to hold the platforms accountable for allegedly causing addiction and mental health problems.

TikTok and Snap, who were also named as defendants in K.G.M.’s lawsuit, reached settlements before the trial. They remain defendants in a series of similar lawsuits expected to go to trial this year.

Matt Bergman, founding attorney of the Social Media Victims Law Center — which is representing hundreds of plaintiffs in state and federal proceedings — said the jury’s decision Wednesday “establishes a framework for how similar cases across the country will be evaluated and demonstrates that juries are willing to hold technology companies accountable when the evidence shows foreseeable harm.”

“Families pursuing justice in other jurisdictions can now point to this outcome as proof that these claims deserve to be heard and taken seriously,” Bergman said in a statement.

Lanier told NBC News in an interview that this was the most difficult case he’s tried in his 42 years as a lawyer.

“I think the jury understood that they were the very first case in the history of our country to look at social media addiction, and they wanted to leave no question, but that they seriously considered the evidence,” Lanier said. “So they took forever, then they looked carefully at each of the questions and answered everyone was, yes, guilty.”

California Attorney General Rob Bonta also weighed in on the Los Angeles and New Mexico verdicts, writing in an X statement that California “looks forward to holding Meta accountable in our own upcoming August trial in the Bay Area.”